The proxy that represents a planet currently supports three components that make up its visuals:
The cm, ng, and add textures that you created will be applied to this opaque sphere.
The cloud proxy definition you reference will have its texture applied to this transparent sphere.
The additive glow drawn around the proxy planet; can be tinted and resized.

The planet proxy is defined by the associated EntityComponent definition which has a SubtypeId which begins with "PlanetProxyType_" and ends with the name of the planet (SubtypeId). You can also define proxies that do not associate with any particular planet type. In this case, they can be manually selected as proxy override options in the Editor, but will not be applied to any planet automatically. Additionally, you can assign extra planet ids to automatically associate with this proxy by using the "PlanetDefaults" option listed below.

Definition Values
"PlanetDefaults"
A list of additional planet subtypeids that the proxy should be automatically associated with (i.e. if you have a planet that has a normal and water mod variant and want the proxy to be assigned to both by default). The list is separated by commas (so something like "PlanetDefaults: EarthLike,Mars,Alien" would work). Can also be used if you want the name of the proxy skin (as it shows up in the Editor) to be different from the planet name.

"PlanetTexture_cm"
Local path from the mod root to the colormap texture. This should be set to point to the "_cm.dds" file you copied to the Textures folder.

"PlanetTexture_ng"
Local path from the mod root to the normal/gloss texture. This should be set to point to the "_ng.dds" file you copied to the Textures folder.

"PlanetTexture_add"
Local path from the mod root to the additive texture. This should be set to point to the "_add.dds" file you copied to the Textures folder (if you have one).

"CloudTexture"
Name of cloud definition that the proxy uses. Can be set to any "CloudProxyType_" definition even if it isn't in your mod. Remove this line if the planet does not have a cloud layer visible from space.

"CloudSizeMult" Default = 1
Multiplier to the size of the proxy cloud layer.

"CloudRotationPeriod" Default = 7200
Number of seconds that it takes for the proxy cloud layer to rotate around the planet once. Default is 7200 seconds (2 hours).

"CloudNormal" Default = (-0.2, 1.0, 0.2)
Up direction for cloud model. Clouds will rotate around this axis.

"CloudShowInOrbit" Default = false
If true, cloud layer will stay visible even when player is within the planet's zone and will fade/shrink out with the main proxy model. Normally not necessary since real planet clouds are visible and can replace the proxy clouds after entering the planet's zone.

"AtmoColor"
Color of the proxy planet's atmosphere. Remove this line if the planet does not have an atmosphere.

"AtmoThickness" Default = 1
Size multiplier of the proxy planet's atmosphere. Remove this line if the planet does not have an atmosphere.

"AtmoColorMult" Default = 1
Multiplier to the brightness of the planet's atmosphere. Remove this line if the planet does not have an atmosphere.

"Scale" Default = 1
Multiplier to the overall size of the proxy. Affects all parts of the proxy.

"ScaleFadeMin" Default = 0.7
Modifier to height at which proxy will shrink to and disappear when approaching a planet. You should generally only change this if the planet has deep canyons or pits and the proxy is not shrinking far enough before disappearing. Value must be between 0 and 1.

"EditorIconColor"
Sets the color of the planet icon as it appears in the Solar System Editor.

As you make changes to the proxy definition, you will want to load into a world that has "Real Solar Systems", the planet mod you are making the proxy for, and your local mod that defines the new proxy. The changes you make to the proxy definition will appear upon exiting and reentering the world.

If you make changes to any of the texture files that the proxy points to, you will need to open the "Debug draw" menu with F11 and click the "Reload textures" button to see any changes to the texture files in game. This can be done without leaving and re-entering the world. The textures will NOT automatically reload if you just leave and re-enter the world, so you NEED to click this button or quit and reopen SE entirely.